1. Field of the Invention
The present invention concerns a magnetic field generating device used, for example, in a magnetic resonance imaging apparatus (hereinafter referred to as MRI), particularly for medical use.
2. Description of the Prior Art
MRI is an apparatus adapted to obtain a tomographic image of a portion of a person and provide an image of his or her tissue.
In the magnetic field generating device used for the MRI, it is necessary that the provided gap have a space which is capable of partially or wholly accommodating the person to be diagnosed and it is also necessary to form a stable intense magnetic field, usually of 0.02 to 2.0 T and at an accuracy of less than 1.times.10.sup.-4 in an image pick-up view field in the gap, i.e., in order to obtain a clear tomographic image.
A known magnetic field generating device used for MRI has a construction as shown in FIGS. 12(a) and 12(b) in which a pair of components 1, 1 made of permanent magnets are opposed to each other as a magnetic field generating source, with one end of each of them being secured with a magnetic pole piece 2 and the other ends of them are connected by means of yokes 3, to generate a static magnetic field in a gap 4 defined between the magnetic pole pieces 2 and 2.
An annular protrusion 5 is disposed at the periphery of each of the magnetic pole pieces 2, 2 in order to improve the uniformity of the magnetic field distribution in the gap 4, and it is usually made of a plate-shaped bulk (one piece product) prepared by scraping from magnetic material, such as electromagnetic soft iron or pure iron (refer to Japanese Patent Laid Open Sho 60-88407).
Gardient magnetic field coils 6 disposed in the vicinity of each of the magnetic pole pieces 2, 2 usually comprise three sets of coil groups corresponding to three directions X, Y and Z for obtaining positional information in the gap 4, but they are described simply in the drawing.
When a pulse current is applied to the gradient magnetic field coils 6, a gradient magnetic field changing with time in a trapezoidal waveform can be generated in a desired direction.
When the pulse current is caused to flow in the gradient magnetic field coils 6, since the magnetic pole pieces 2 are composed of the plate-shaped bulk as described above, magnetic the field changes abruptly upon rising and falling of the current to induce an eddy current in the magnetic pole piece 2, 2.
Since the eddy current forms a magnetic field in a direction opposite to the magnetic field formed with the gradient magnetic field coils 6, it takes a long time for the gradient magnetic field to reach a predetermined intensity.
As a means for overcoming the foregoing problem, there has been proposed a magnetic field generating device using, as the magnetic pole piece, a plate-shaped lamination comprising soft magnetic thin films, such as permalloy steel sheets or amorphous steel sheets, which are laminated such that the plane of lamination is in perpendicular to the plane of the magnetic pole pieces, and arranged and integrated in two layers such that the laminating directions are different by about 90.degree. from each other (Japanese Patent Laid Open Sho 61-203605), or a magnetic field generating device using a magnetic powder of high specific resistivity (Japanese Patent Laid Open Sho 63-25907).
However, the construction described above, which is intended to reduce the eddy current, still involves a problem that the magnetic pole pieces are magnetized by the magnetic field formed with the gradient magnetic field coils (GC), and the uniformity of the magnetic field in the gap is distorted by the residual magnetism due to the magnetic hysteresis phenomenon (residual magnetism phenomenon) even after the interruption of the GC pulses due.